PROJECT SUMMARY Anterior shoulder instability (ASI) is one of the most common shoulder ailments in the United States.1 A large proportion of ASI patients undergo surgical stabilization, typically arthroscopic soft tissue repair (i.e. repair of the labrum and/or joint capsule), with approximately 260,000 surgeries of this type performed each year in the United States.2,3 While the majority of these cases are successful, up to 35% are considered clinical failures due to recurrent ASI and/or persistent limited shoulder function.4,5 One of the major reasons for failure of arthroscopic repair in ASI patients is the inadequate evaluation and treatment of bone injuries that occur in this clinical setting, humeral Hill Sachs lesions and anterior glenoid bone loss, (i.e. bipolar bone loss), reported to be the cause of recurrent ASI after surgery in up to 67% of patients.6 MRI plays a crucial role in the preoperative evaluation of patients with ASI, providing important information on the associated soft tissue and bone injuries. Prior studies have defined imaging based thresholds of ?significant? bone loss at the humerus and glenoid that are used to guide the selection of treatment with the location of greatest bone injury typically determining management. However, the utility of these thresholds and the consideration of only one bone injury have come into question with several studies demonstrating conflicting treatment results and patient outcomes using these techniques. More recently, greater emphasis has been put on the potential interaction between these injuries rather than their sizes as the main risk factor for recurrent ASI. To improve the preoperative evaluation of bipolar bone loss, we will develop and optimize a new imaging strategy that consists of an imaging technique, 3D MRI, and imaging measurement tool, the On/Off Track method, that can be easily implemented into clinical practice. 3D MRI reconstructions are created using standard imaging data and minimal post processing time without the cost and radiation associated with other 3D models. The On/Off Track method is a technique that uses simple measurements of bipolar bone loss to assess the potential for interaction between these osseous injuries in order to predict recurrent ASI. We will examine the value of the On/Off Track method on MRI with 3D MRI models in predicting which patients with a history of ASI will be considered a failure within 24 months of surgery. We will also determine the most cost-effective pre-operative imaging strategy for patients with a history of ASI. This study will establish a new cost-effective MRI strategy for the assessment of bipolar bone loss in the patient with ASI, which can be used to select the most appropriate initial treatment and improve patient outcomes.